Field of the Invention
The present invention relates to an audio output circuit that drives a speaker or headphones.
Description of the Related Art
An audio output circuit is employed in order to amplify a weak audio signal so as to drive an electroacoustic conversion element such as a speaker, headphones, or the like. FIG. 1 is a circuit diagram showing a configuration of an audio output circuit 100r including a Class D amplifier. The audio output circuit 100r includes a Class D amplifier 10, a driver 20, a pulse modulator 30, and an amplifier 40. The amplifier 40 amplifies an input audio signal SIN. The pulse modulator 30 generates a pulse signal S2 that is pulse modulated so as to have a duty ratio that corresponds to the audio signal S1 amplified by the amplifier 40. The driver 20 drives the Class D amplifier 10 according to the pulse signal S2.
A high-side transistor MH and a low-side transistor ML of the Class D amplifier 10 switch on and off in a complementary manner so as to output, via an output terminal OUT, a driving voltage SOUT that switches between a power supply voltage VDD and a ground voltage VGND. The driving voltage SOUT is input to an electroacoustic conversion element 102 via a filter 104.
The present inventors have investigated the efficiency of the audio output circuit 100r shown in FIG. 1, and have come to recognize the following problems.
The efficiency η of the Class D amplifier is represented by the following Expression.η=POUT/PIN 
Here, POUT represents electric power supplied to the electroacoustic conversion element 102 that functions as a load, and PIN represents electric power input to the audio output circuit 100. PIN is represented by the following Expression.PIN=PSW+PSHT+PON+POUT 
Here, PSW represents switching loss, PSHT represents short-through loss, and PON represents loss due to on resistance.
The switching loss PSW is electric power required to switch on and off the high-side transistor MH and the low-side transistor ML of the Class D amplifier 10. Specifically, the switching loss represents electric power required to charge and discharge the gate capacitance CG. Thus, as the gate capacitance CG becomes larger, i.e., as the transistor size becomes larger, the switching loss PSW becomes more dominant.
In such an audio output circuit 100r, the transistor size of the Class D amplifier is determined so as to provide favorable distortion characteristics in its maximum output operation. The switching loss PSW is constant regardless of the magnitude of the output power POUT of the Class D amplifier. Accordingly, in a small POUT range, the ratio of PSW with respect to PIN is dominant. Thus, in this range, such an arrangement has a problem of reduced efficiency.
It is rare for such an audio output circuit 100r to be used at its maximum output level. In actual use, such an audio output circuit 100r is used in its medium to lower range. Alternatively, if the time period of a silent state becomes long, improvement of the efficiency η in the low output state is effective for reducing power consumption.